(1) Field of the Invention
The present invention relates to a fiber-optic cable, in particular for use in a borehole, according to the preamble of claim 1. The fiber-optic cable is hereinafter also referred to as the wireline cable.
Definitions: The terms light, optical radiation or optical signal used in the following refer to electromagnetic radiation in the optical spectral range, in particular from XUV to FIR. Accordingly, within the context of this application, a light waveguide is used as a transmission medium for electromagnetic radiation in the optical spectral range.
(2) Description of Related Art
In oil and natural gas wells, multi-functional mobile tools (“tractors”) are used for sensory as well as for maintenance purposes. These are conventionally supplied with energy over conventional wireline cable structures. Hitherto, wireline cables with optical waveguides integrated in a stainless steel tube are inadequate in practical use with distributed fiber-optic sensors because different elongation coefficients of cables and tubes during insertion and extraction of the cable, as well as extreme longitudinal pulling forces and torsion can occur when driving the tractor. This can cause local mechanical deformations and tearing of the tube. An alternative version without a tube enclosing the optical waveguide for hermetic protection is also known. The resulting proposed structures can indeed allow greater elongation. However, it can be expected that, for example, irreversible elongation can occur when the longitudinal tension and torsional forces are applied, which cause local stress (locally increased attenuation) on the sensor fibers. In addition, by eliminating a tube enclosing the optical waveguides, the waveguides are not hermetically protected, which is disadvantageous with respect to accelerated aging effects (hydrogen ingression, high temperatures in the borehole from 200° C. to 300° C. or higher) in this application.
Both of the above-mentioned variants have the disadvantage that mechanical forces (longitudinal tensile forces and torsion) during continuous operation of the wireline cable may cause temporary or permanent locally different impairment of the sensory properties of the optical waveguides as fiber-optic sensors distributed over various locations. This affects the quality of calibration and measurement resolution of fiber-optic method for measurement of physical quantities.
Wassink, Sandra, EX-Journal 2011, page 34-41, “Wireline— . . . ” describes the advantages of wireline technology (from page 38) in land-based oil and natural gas production. The application “Wireline” implements technical systems which allow measurements within a borehole during ongoing production.
WO 2011/037974 A2 describes the wireline technology with an extension to additional maintenance tasks within a borehole. A propulsion unit (tractor) travels along the subterranean borehole to perform different tasks (maintenance, measurement), wherein power is supplied and data are transferred via the wireline cable.
The tractor and the wireline cable remain permanently in the borehole and, if necessary, the tractor can be pulled back with the wireline cable to the starting point of the borehole. The optical system (measurement) of physical variables such as temperature is simultaneously implemented by using optical fibers within the wireline cable.
WO 2011/037 974 A2 addresses in this context the requirements for the torsion properties, the cable weight and the frictional resistance of the wireline cable. For this purpose, different design solutions of wireline cables are illustrated in this document, which have improved torsion properties (torque balanced) compared to the existing wireline cables.
To reduce the frictional resistance, an additional smooth outer jacket is implemented on the wireline cable. However, the question remains to which extent smooth characteristics of a thin plastic jacket can be maintained under adverse conditions. Likewise, there is the risk that the jacket can wear off and/or tear.
In U.S. Pat. No. 7,324,730 B2, it is demonstrated that the use of stainless steel tubes for the protection of optical fibers in the wireline cable application is not or only barely suitable. Due to high cable elongations of wireline cable in an application, stainless steel tubes can be at risk of deformations. In the worst case, the optical fibers within the steel tubes are also damaged.
U.S. Pat. No. 7,324,730 B2 discloses novel constructive solutions without metallic tubes as direct protection for the optical fibers. These solutions are intended to prevent damage to the optical fibers with greater cable elongation.
According to the aforementioned findings, the structures disclosed in WO 2011/037974 A2 do not disclose additional constructive features that would be able to adequately protect a stainless steel tube at high elongations of the wireline cable against deformation or damage.
Accordingly, no suitable precautions for the safe use of stainless steel tubes in the wireline cable application exist in conjunction with the portable use of a tractor. It is therefore desirable to permanently enable the distributed fiber-optic measurement of physical parameters along the cable line in the above application with a suitable construction of a wireline cable in a tube, for example made of stainless steel, and at least one integrated optical waveguide and to minimize the aforementioned disadvantages.
A fiber-optic cable of the type mentioned above is known from U.S. 2006/0120675 A1 The cable disclosed therein includes a stainless steel tube with an optical fiber disposed therein. In addition, a reinforcement layer made of aramide fibers is arranged outside of the tube and Teflon layers outside of the reinforcing layer to reduce friction.